When did the moon form?In part 1 of this story, I discussed how planets form from the early solar ne

When did the moon form?In part 1 of this story, I discussed how planets form from the early solar nebula and how the formation of cores is a key step in their chemistry (see here: https://www.facebook.com/TheEarthStory/posts/669201963140851 ). When cores form, certain elements are taken out of the mantle almost completely and locked away in the core for almost all time.This process happened many times in the early solar system, even on objects as small as the asteroid Vesta. Every time it happened, it left a chemical fingerprint. The element Hafnium has a radioactive isotope that decayed to Tungsten early in solar system history. Tungsten is siderophile – when cores form, almost all the Hafnium stays in the mantle while almost all the Tungsten goes into the core. Therefore, almost every atom of tungsten in the mantle should have formed after core-formation and counting those atoms can give a date.Unfortunately, it’s never been that simple. The Earth’s core probably formed in several steps – cores formed on the objects that went into Earth, and when those objects collided, the cores splashed together and mixed in with the mantles. On top of that, other elements have been added back into the mantle after the cores finally formed since meteorites do continue hitting the earth – a process we call adding a “Late Veneer”.That brings me to this paper regarding Moon formation. The giant impact that formed the moon was likely the last giant impact on Earth – the last time anything was added to Earth’s core. That impact added so much energy that part of the planet vaporized and the mantle melted, forming a magma ocean probably about 2000 kilometers deep.If, at that time, all the iron-loving elements in the mantle were taken to the core, then everything in the mantle today was delivered to the planet after that impact. The scientists from France came up with a novel idea – to date the Earth, they could simply count how abundant those elements are in Earth’s mantle.To do this, they simulated how the solar system evolved. To make the current mantle, the moon-forming impact had to happen when there was still some mass remaining in the solar system – the planets can’t have eaten it all beforehand. They modeled how the planets formed and found that the moon-forming impact must have happened around 95 million years after the start of the solar system. With their models, that is the only way to make the Earth’s mantle as we see it today.Is there any reason why this analysis might not give the right age? Well, there are a couple. Even the moon-forming impact likely didn’t melt Earth’s entire mantle; that’s one of the issues that complicated the earlier measurements. If the whole mantle didn’t melt, then it’s possible to have a lower mantle which retained some elements, altering the simple calculation. It’s also worth noting that this group’s models for accretion in the early solar system differs from other groups; the other models would put the same constraint much later, as late as 150 million years, a number so late it’s actually difficult to explain with the context of moon rocks in our collection.This is an interesting attempt to explain the timing of moon formation, but it’s only one part of the picture. To give a full picture, we need a structure that explains the chemistry and isotopic composition of the earth and the moon all at once in the context of the early solar system; all complexities scientists are working on to this day.-JBBImage credit: http://en.wikipedia.org/wiki/File:Artist%27s_concept_of_collision_at_HD_172555.jpgRead more:http://www.iflscience.com/space/dating-moon -- source link

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